1. Field of the Invention
This invention relates to laser shock peening of leading and trailing edges of gas turbine engine blades and, more particularly, for laser shock peening airfoil leading and trailing edges of blades of integrally bladed rotors and disks such as found on fan and compressor blades to form laser shock peened regions having localized compressive residual stresses imparted by laser shock peening.
2. Discussion of the Background Art
Gas turbine engines and, in particular, aircraft gas turbine engines rotors operate at high rotational speeds that produce high tensile and vibratory stress fields within rotating blades, such as fan and compressor blades and makes blades susceptible to foreign object damage (FOD). Vibrations may also be caused by vane wakes and inlet pressure distortions as well as other aerodynamic phenomena. This FOD causes nicks and tears and hence stress concentrations in leading and trailing edges of blade airfoils. These nicks and tears become the source of high stress concentrations or stress risers and severely limit the life of these blades due to High Cycle Fatigue (HCF) from vibratory stresses. These and other operational phenomena lead to incipient cracking and material failure of portions of objects such as along airfoil edges.
Therefore, it is highly desirable to design and construct longer lasting fan and compressor blades, as well as other hard metallic parts, that are better able to resist both low and high cycle fatigue and that can arrest cracks than present day parts. The above referenced U.S. Patent Applications are directed towards this end. They teach to provide an airfoil of a blade with regions of deep compressive residual stresses imparted by laser shock peening on at least a radially extending portion of leading and/or trailing edge surfaces of the blade.
The region of deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a workpiece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and, thereby, harden the workpiece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled xe2x80x9cMethod and apparatus for truing or straightening out of true work piecesxe2x80x9d. The prior art teaches the use of multiple radiation pulses from high powered pulsed lasers and large laser spot diameters of about 1 cm to produce shock waves on the surface of a workpiece similar to the above referenced Patent Applications and U.S. Pat. No. 3,850,698, entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477, entitled xe2x80x9cLaser shock processingxe2x80x9d; and U.S. Pat. No. 5,131,957, entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d.
Laser shock peening leading and trailing edges of gas turbine engine fan and compressor blades and stator airfoils have been disclosed in U.S. Pat. No. 5,591,009, entitled xe2x80x9cLaser shock peened gas turbine engine fan blade edgesxe2x80x9d; U.S. Pat. No. 5,531,570, entitled xe2x80x9cDistortion control for laser shock peened gas turbine engine compressor blade edgesxe2x80x9d; U.S. Pat. No. 5,492,447, entitled xe2x80x9cLaser shock peened rotor components for turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329, entitled xe2x80x9cAdhesive tape covered laser shock peeningxe2x80x9d; and U.S. Pat. No. 5,674,328, entitled xe2x80x9cDry tape covered laser shock peeningxe2x80x9d, all of which are assigned to the present Assignee. The laser shock peening methods disclosed in these patents include ways of laser shock peening airfoil leading and trailing edges using circular cross-sectional laser beams that form circular laser spots on the edges.
U.S. Pat. No. 5,492,447 discloses laser shock peening an interior annular region by orbiting a laser beam at an oblique angle to the interior surface of the interior annular region. U.S. Pat. No. 5,911,890 teaches controlling the incident angle of the laser beam applied to the workpiece and controlling the shape of the beam with lenses, polarizers, and particular transparent overlay geometries. The apparatus and methods disclosed includes use of structure for controlling the position and incident angle of the laser beam and controlling the polarization and/or the shape of the incident impact area, based on such incident angle. The patent teaches that an oblique incident angle laser beam having a circular cross-section causes the shape of the impact spot to be elliptical and that the consequences of such a change of the incident spot shape necessarily changes the energy density applied to the workpiece. The patent further teaches that the energy density per unit area compared to other areas on the same surface creates a possibility of non-uniformly working the material, thereby, possibly losing some of the benefits of laser shock peening. This non-uniformity of energy application to a workpiece may cause severe problems, particularly, when hitting a workpiece from opposite sides at the same time, as used with a split beam laser system. Such opposing hits are sometimes needed on workpieces of thin cross-section, such as disks, blades, and other workpieces of different geometries. In conventional split beam processing, there is a possible effect of not having the laser processed portions on the opposite sides of the workpiece worked identically, and at the same time when elliptical spots are utilized. Such non-uniform working of the workpiece may cause over or under working of the material or distortion of the workpieces, thereby, not achieving the goals of laser shock processing. Furthermore, based upon the oblique angle along with the particular transparent overlay material utilized, polarization issues regarding the reflection of the laser beam from the surface of the transparent overlay layer can possibly degrade and reduce the energy applied to the workpiece. Thus, in general, the patent concludes not to use elliptical laser spots but rather a more complicated apparatus and method to significantly reduce the non-uniformity of the applied energy to a workpiece by modifying the shape of the applied laser energy pulse. The patent teaches to pass the pulse of energy through a lens to reform the shape of the incident area on the workpiece, to counteract geometric effects created by the workpiece surface orientation on the incident area shape.
Manufacturers are constantly seeking methods to reduce the time, cost, and complexity of laser shock peening processes and, it is to this end that the present invention is directed and, more particularly, to avoid the use of apparatuses and methods to reshape laser beams used in laser shock peening.
A first exemplary embodiment of the invention is a method for laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blade with circular cross-section laser beams, firing the laser beams, at an oblique angle with respect to the surfaces so as to form elliptical shaped laser spots on the surfaces, and overlapping adjacent elliptical shaped laser spots and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces. The elliptical shaped laser spots have major axis extending away from the edge and over the surfaces and transverse minor axis and, in a more particular embodiment of the invention, the elliptical shaped laser spots overlap by about 50% and the laser spots extend over the edge. Relative movement between the laser beams and the surfaces is effected while the laser beams are being fired.
A second exemplary embodiment of the invention is a method of laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element circumscribed about an axis of rotation and having an annular space between adjacent axially spaced apart forward and aft and rows of blades. This method laser shock peens leading or trailing edges that border the space by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blades in one of the rows with circular cross-section laser beams, firing the laser beams at a first oblique angle with respect to the surfaces so as to form elliptical shaped laser spots on the surfaces and at a second oblique angle with respect to the axis wherein the second oblique angle is sufficient to clear blades in the adjacent row of blades, and overlapping adjacent elliptical shaped laser spots and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces.
A third exemplary embodiment of the invention is a method of laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blade with circular cross-section first and second laser beams respectively, firing the first laser beam at an oblique angle with respect to the pressure side surface so as to form elliptical shaped laser spots on the pressure side surface, firing the second first laser beam at about a normal angle with respect to the suction side surface so as to form circular shaped laser spots on the suction side surface, and overlapping adjacent elliptical shaped laser spots and circular shaped laser spots respectively and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces.
A fourth exemplary embodiment of the invention is a method of laser shock peening leading or trailing edges of gas turbine engine blades mounted on a rotor element circumscribed about an axis of rotation and having an annular space between adjacent axially spaced apart forward and aft rows of blades, wherein the edges being laser shock peened border the space. This method laser shock peens leading or trailing edges that border the space by simultaneously laser shock peening pressure and suction side surfaces along one of the edges of the blades in one of the rows with circular cross-section laser beams, firing the first laser beam at a first oblique angle with respect to the pressure side surface so as to form elliptical shaped laser spots on the pressure side surface and at a second oblique angle with respect the axis wherein the second oblique angle is sufficient to clear blades in the adjacent row of blades, firing the second first laser beam at about a normal angle with respect to the suction side surface so as to form circular shaped laser spots on the suction side surface and at a second oblique angle with respect the axis wherein the second oblique angle is sufficient to clear blades in the adjacent row of blades, and overlapping adjacent elliptical shaped laser spots and firing the laser beams with sufficient energy to form regions having compressive residual stresses imparted by the laser shock peening extending into the blade from the surfaces.
The present invention is a faster and more cost efficient method to laser shock peen surface portions of leading and trailing edges gas turbine engine blades mounted on rotor sections or elements. An integrally formed bladed rotor section is also referred to as an integrally bladed rotor (IBR) or integrally bladed disk (BLISK) having two or more spaced apart rows of blades integrally mounted or formed on a drum rotor or disk respectively. The blades are designed to operate in high tensile and vibratory stress fields and laser shock peening enables the blades to better withstand fatigue failure due to nicks and tears in the leading and trailing edges of the blades and, therefore, have an increased life over conventionally constructed blades. A laser shock peening production line may be set up and operated less expensively compared to those suggested in the prior art, due to lower capital outlay. The line should be less complex to develop, design, and construct because the method for the present invention uses direct laser beams without intervening lenses for laser beam shaping.